Multi-mode fiber coupler, system and associated methods

ABSTRACT

An optical coupler reduces differential mode delay in a fiber by reducing an amount of light incident on the fiber in a region in which the refractive index is not well controlled. This region of the fiber is typically in the center of the fiber. The optical coupler directs light away from the this region and/or provides a high angle of incidence to any light on this region. A diffuser may be used to reduce sensitivity of the coupler to any fluctutations in the output of the light source. The optical coupler does not need to be offset from the center of the multi-mode coupler. A phase function of an azimuthal mode of the fiber may be imposed on the light beam so that a substantial null on axis is maintained even after propogation of the light beam beyond the depth of focus of the coupler. A diffractive element generating a beam which propogates in a spiral fashion along an axis allows the shape of the beam to be maintained for longer than a depth of focus of the diffractive element.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 toProvisional Application No. 60/101,367 filed on Sep. 22, 1998, theentire contents of which are hereby incorporated by reference for allpurposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a coupler for coupling light to amulti-mode fiber.

2. Description of Related Art

It is very difficult to manufacture a multi-mode fiber with good controlover the index of refraction in the center of the fiber. If the lightcoupled to the fiber excites some modes that propagate mostly in thecenter of the fiber and other modes which do not propagate mostly in thecenter of the fiber, very different propagation times for these modesmay result. This is referred to as differential mode delay. Differentialmode delay tends to spread out the pulse length of signals and reducethe effective bandwidth of the fiber.

Modes which propagate mostly in the center of the fiber are the lowerorder fiber modes, i.e., modes having small propagation angles thatstrike at or near the center of the fiber. These lower order modes spendmost of the time in the center of the fiber, tend to travel straightdown the fiber and the shape of these modes does not change much as theypropagate. Therefore, in order to reduce differential mode delay, anylight which enters near the center of the fiber needs to be incident atan angle which is large enough not to excite lower order modes, but notso large that the critical angle is exceeded and the light fails to becoupled or no light should be input to the center of the fiber.

One current solution involves coupling light into single mode fiberswhich are then positioned off-axis relative to the multi-mode fiber.Single mode fibers have a much smaller core than multi-mode fibers, socan be used to provide light at specific positions on the endface of themulti-mode fiber. However, single mode coupling is more expensive thanmulti-mode coupling and the additional coupling step leads to anincrease loss in light. Further, while no light enters the fiber of thecenter for this configuration, the light will still cross the fiber axisas it propagates, thus increasing the differential mode delay.Additionally, ferrules or other structures housing the multi-modefiber:to a single mode fiber junction are not readily available and mustbe developed specifically for that purpose.

Another solution is to use a vertical cavity surface emitting laser(VCSEL) excited to radiate in a ring mode. The operation of the VCSEL inradiation modes other than the lowest order have less predictable fluxdistributions than in the lowest order mode, in which the distributionmore closely approximates a Gaussian profile. Further, there will stillbe some power in the lower order modes of the VCSEL. Additionally, suchoperation of the VCSEL often requires a higher current to drive thesource into the higher radiation modes.

SUMMARY OF THE PRESENT INVENTION

The present invention is therefore directed to a multi-mode fibercoupler which substantially overcomes one or more of the problems due tothe limitations and disadvantages of the related art.

More specifically, it is an object of the present invention to reducedifferential mode delay while coupling light into a multi-mode fiber.

It is another object of the present invention to efficiently couplelight into a multi-mode fiber without requiring an offset fiber oradditional beam shapers for the light source.

It is another object of the present invention to use an optical elementto direct light away from the center of the fiber.

It is a further object of the present invention to provide coupling to amulti-mode fiber which is relatively insensitive to variation in lightoutput from a light source.

It is yet another object of the present invention to provide adiffractive element which generates a beam propagating in a spiralfashion.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will bedescribed with reference to drawings, in which:

FIG. 1A is a schematic view of one embodiment of the present invention;

FIG. 1B is a schematic view of another embodiment of the presentinvention;

FIG. 2 is a radiation profile of light traversing an embodiment of theoptical element 12 of FIG. 1A or 1B;

FIG. 3 shows another embodiment of the optical element of the presentinvention;

FIGS. 4A, 4B and 5 show other embodiments of the optical element and/orbeam shaper of the present invention;

FIG. 6 shows the integration of a light source and the coupler of thepresent invention;

FIG. 7 shows the integration of the multi-mode coupler of the presentinvention with the fiber;

FIG. 8 shows the integration of the multi-mode coupler of the presentinvention with the light source and the fiber;

FIG. 9 shows the integration of the multi-mode coupler of the presentinvention with the light source,and the fiber on a silicon bench;

FIG. 10 shows the integration of the multi-mode coupler of the presentinvention with a light source, a fiber and a light source power monitor;

FIGS. 11A-11C illustrate a diffractive element and associated.characteristics of a spiral generator of the present invention; and

FIG. 12 are comparative plots of encircled incident energy for aGaussian beam and a donut shaped spiral beam.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1A and 1B schematically illustrate different configurations of themulti-mode coupler of the present invention. A light source 10, such asa VCSEL, an edge emitting laser or a single mode fiber, outputs lightwhich is incident on a beam shaper 13 which shapes the beam and anoptical element 12 together forming the coupler. The light is thensupplied to a core 16 of a multi-mode fiber 14.

The end face of the fiber is typically located near the image plane ofthe optical system as determined by the focal length of the beam shaper13 and the object distance, i.e., the distance from the light source 10to the beam shaper 13. If the fiber 14 is placed substantially furtherthan a depth of focus away from the image plane, then the beam will bebigger than the core 16 of the fiber 14, resulting in less light beingcoupled to the fiber 14.

The optical element 12 may direct light away from a center of the core16 of the fiber 14 by, e.g., increasing the angle of light in the centerof the beam so that light in the center will be incident on the outeredges of the core 16 of the fiber 14 or by delivering no light to thecenter of the core. Thus, either no light is delivered to a center 16 ofthe fiber 14 or any light which is incident on the center 16 of thefiber 14 will be incident at a high enough angle to be coupled into thedesired higher order modes.

In addition to the optical element 12, a beam shaper 13 may be providedin either embodiment. The beam shaper 13 may be integrated with theoptical element 12 on a same surface or on an opposite surface of thesame structure. The beam shaper 13 may also be closely spaced to theoptical element 12. As shown in FIG. 1B, the beam shaper 13 may beplaced at a specific distance from the light source 10, with the opticalelement 12 being very close to or even flush with the fiber 14. Eachembodiment has attendant advantages and disadvantages as discussedbelow.

The beam shaper 13 performs a one-to-one mapping from the input plane tothe output plane thereof. The performance of the beam shaper may beevaluated using ray tracing. Typically, the beam shaper 13 is used forfocusing the beam output by the light source 10, which will usually beon the order of a several hundred microns in the plane of the beamshaper, to a diameter which is smaller than the diameter of the core,which is usually on the order of 50 microns. If the beam shaper is alens, theoretically, light is focused to a point. But in reality, if thelight incident on the lens has a Gaussian profile, the light output fromthe lens will still have a Gaussian profile. Another useful beam shaperfor the coupling of the present invention is a super-Gaussian element. Asuper-Gaussian element converts an input beam of a particular intensitydistribution into a beam with a super-Gaussian distribution, therebyproviding a focused output beam having a flatter peak and a much fasterfall off to zero than a normal Gaussian beam. Thus, such a beam has afairly uniform power distribution across the peak, pushing more power tothe edges and leaving less in the center as compared to a normalGaussian. When the optical element serves as an optical profile alteringelement, the beam shaping and the optical element may be formed on asingle surface.

While the ratio between the distance from the light source 10 to thebeam shaper and the distance from the beam shaper to the fiber 14 shownin FIGS 1A and 1B, in which an edge emitting laser is used as the lightsource, is typically 2:1, when using a VCSEL as the light source 10,this ratio is typically closer to 1:1. The actual ratio will depend onthe numerical aperture of the source and the numerical aperture of thefiber. Further, depending upon the desired coupling, the beam incidenton the fiber may be smaller than the core or larger than the core. Formost applications, source and fibers, the ratio will be between 1:4 and4:1.

There are three primary design approaches for achieving the desiredshaping by the optical element 12. The radiation profile of light havingtraversed a first embodiment of the optical element 12 is shown in FIG.2. As can be seen in FIG. 2, the radiation profile has been altered bythe optical element 12 to be bimodal. This bimodal distribution isGaussian shaped for each peak, each peak being centered on an absoluteangular value between zero and θmax, where θmax is the critical anglefor the multi-mode fiber 14.

In the first design, the optical element 12 is a diffractive diffuserwhich diffuses, i.e., substantially each point of light incident on thediffuser substantially contributes to substantially every point of lightin the output plane, the light into the desired angular distribution.The angles will all be less than the critical angle for the fiber 14.Thus, if there is change in the output profile of the light from thelight source, which is of particular concern when using a VCSEL as thelight source 10, the coupling to the multi-mode fiber will not beaffected. Additionally, if the diffractive diffuser does not alsoprovide collimation or focusing to the light, precise alignment of thediffractive diffuser is not needed.

A diffractive diffuser may be formed by setting the fast Fouriertransform (FFT) to be a ring, i.e., set the fringe period of thediffractive between the two values bounding the ring. In order for thediffractive diffuser serving as the optical element 12 to functionproperly, it must be positioned at least more than a width of the core,preferably at least three to five times the width of the core, away fromthe fiber. This placement ensures that substantially every point oflight incident on the diffuser substantially contributes tosubstantially all of the pattern incident on the fiber. Such a Fouriertransform diffractive diffuser may be realized in accordance with U.S.Pat. No. 5,850,300, which is hereby incorporate by reference in itsentirety.

The diffractive diffuser preferably alters the angular distribution ofthe light into any desired angular distribution which will efficientlycoupler the light into the higher order modes of the multi-mode fiber.This desired angular distribution will typically be a ring, an annulusor a grid of N spots, but may be any other desired angular distributionfor a particular multi-mode fiber. For example, a radial grating may beprovided which sends a significant portion of the light, e.g., 80%, intothe ±1 order and randomly varies the period to provide the range ofdesired angles radially away from the center. Further, a ring or amultipole of N spots, e.g. a quadropole of 4 spots, where N is aninteger greater than or equal to one, may be realized by providing agrating to create spots located at r_(N), where r_(N) is a distance fromthe center to the spot. Additionally, the diffractive diffuser may be abinary element which splits the light into two beams directed to theperiphery of the fiber core.

While a Fourier transform diffractive diffuser as described above isuseful when employing a light source having an unstable output beamprofile, this diffractive diffuser is difficult to use in theconfiguration of FIG. 1B, since the optical element 12 is too close tothe fiber 14 for a Fourier transform diffractive diffuser to create, forexample, a ring on the end face of the fiber. If the optical element 12is more than a few wavelengths away from the end of the fiber, adiffuser serving as the optical element 12 will function properly.Additionally, diffusers often have lower efficiencies than other opticalelements. The following two designs may be used with eitherconfiguration of FIGS. 1A and 1B.

A generic embodiment of the second design is illustrated in FIG. 3. Ascan be seen therein, the optical element 12 is composed of a centralregion 18 and a peripheral region 20. The central region 18 and theperipheral region affect the beam incident thereon differently. Thesedifferent regions may be discretely different, include subregions ofdifferent functioning, and/or may continuously vary the treatment of thelight from the center to the periphery. For example, the central region18 deflects the light incident thereon away from the center. Theperipheral region 20 may not affect the light incident thereon at all,or it may be designed to, for example, collimate the light incidentthereon. Using such an element allows the light in the center of thebeam which would have been incident on the center of the fiber to bedeflected away to edges of the fiber, while not imposing an increase inthe angle on the light near the edge of the beam which would already beincident upon the desired portion of the fiber. Alternatively, althoughnot as efficiently, the central region may simply block the lightincident thereon to form the desired ring shape.

A specific embodiment of the second design is illustrated in FIG. 4A.The optical element 12 provides a one to one mapping of each point tothe fiber, while continuously varying the element encountered by thelight from the center to the periphery. A converging portion 50 of thecoupler 12 converges, i.e., reduces the incident angle, of light at theouter edge of the beam. A diverging prism 52 of the optical element 12diverges, i.e., increases the angle of the light, of light in the centerof the beam to prevent light from hitting the center of the fiber.Another specific embodiment of one to one mapping is shown in FIG. 4B inwhich a diverging portion 54 is located in the center of the opticalelement again to diverge light in the center of the beam. FIGS. 4A and4B are radially symmetric.

Another specific embodiment of the second design is shown in FIG. 5.FIG. 5 is a cross-section of a prism. If this cross-section is used toform a linear prism, such that there is a variation in thickness alongthe axis coming out of the plane of the page, two spots will begenerated in the image plane of the system. When a linear prism iscombined with a lens function, the cross-section will look like FIG. 4B,but will not be radially symmetric, since the linear prism is notradially symmetric. If the cross-section in FIG. 5 is rotated radiallyto form a radial prism, a ring will be generated in the image plane ofthe system. If the radial prism is combined with a lens function, thecross-section will look like FIG. 4B and will be radially symmetric.

In the embodiments of the second design, light near the edge of the beamcan be mapped to the edge of the fiber with little or no increase in theangle. Light from the center of the beam can be mapped to the edges ofthe fiber. Where the optical element 12 is illustrated as a refractiveelement in the embodiments of the second design, the optical element 12may be designed as a diffractive element using the known diffractiveapproximation of the refractive element, either as a continuousdiffractive or as a discrete diffractive. Preferably, the diffractiveelements are computer generated holograms.

The same effect as provided by configurations of the second design maybe realized by providing an optical element having diffuser patcheshaving finer features and/or smaller periods closer to the center andlarger features and/or larger periods towards the periphery or nothingat the periphery. At the edge of the element the light is not effected,or has a small increase in angle, and the light at the center isdiffused to increase the angle of light towards the center. As long asthe diffuser patches are distributed on the optical element so that itdoes not treat the center and the periphery in the same manner, e.g., adiffuser only at the center or a gradient diffuser, the diffuser patchesmay be used next to the end face of the fiber, such as shown in FIG. 1B.Such diffusing patches may also be multiplexed with any desired lensfunction.

Further, while the embodiments of the second design have been discussedwith reference to the optical element 12, the second design may also beused as the beam shaper with the optical element of the first design orthe optical element of the third design, discussed below. Further, whenusing a diffractive diffuser which splits the light into two beamsdirected to the periphery of the fiber core, this element does not haveto be unitary, but may be split into half. In such a configuration, thetwo elements serve as a beam shaper, with one half mapping the lightincident thereon to one point and the other half mapping the lightincident thereon to another point.

The above discussion has assumed that the ideal radiation pattern forcoupling light into the fiber is a ring. Generally, the ideal radiationpattern, and hence the desired angular distribution, will be a functionof the properties of the fiber, i.e., where propagation is mostefficient. The design of the coupler for achieving the desired angulardistribution will also depend on the radiation profile output by thelight source used to illuminate the fiber.

Any of the above designs may be integrated with other optical functions,such as collimation, in a single element, as shown in FIGS. 4A, 4B and5. For the integration of the coupler with additional opticalfunctioning, the additional functioning may be multiplexed with theshaping function, as disclosed in commonly assigned, co-pendingapplication U.S. patent application Ser. No. 09/296,397 filed Apr. 23,1999, entitled “Diffusing Imager and Associated Methods” which is herebyincorporated by reference in its entirety. Further, any of the abovedesigns may be integrated with the other elements of the lightsource/fiber system, including further optical elements as discussedbelow.

FIG. 6 illustrates the integration of the light source 10 with thecoupler 12. In the specific example shown in FIG. 6, the light source 10is an edge emitting laser mounted on a substrate 21. The light from thelaser is directed onto the coupler 12 mounted on a substrate 23 by areflective surface 28 formed on a block 25, which also separates thewafers 21, 23. The block 23 may be formed from the substrate 21. If avertical cavity surface emitting laser is employed, the reflectivesurface is not needed. Also, while a diffractive embodiment of thecoupler 12 is shown, a refractive embodiment could similarly beintegrated. The coupler 12 is preferably created lithographically and isintegrated with the laser on a wafer level.

FIG. 7 illustrates the integration of the optical fiber 14 with a rod 22containing the coupler 12. Both the fiber 14 and the rod 22 are housedwithin a ferrule 24 or other housing containing the fiber. The fiber andthe rod are bonded together with an adhesive 26. Alternatively, theintegrated light source-coupler shown in FIG. 6 can be provided in thehousing with the fiber 14. Additionally, if the beam from the lightsource 10 needs to be expanded, an expanding element 27 may be providedbetween the light source and the diffractive element 12. Forcompactness, the expanding element 27 advantageously will be a GRIN lensor a rod lens. Depending on the size of the beam output by the lightsource 10, the expanding element 27 could just provide distance for thebeam to expand.

FIG. 8 illustrates the integration of the light source 10 with thecoupler 12 and the fiber 14 on a wafer level. Wafers 30 and 34 arebonded together via posts 32, which provide a predetermined separationbetween the wafers. The light source 10 may be bonded directly to thewafer 30 or may be mounted on an additional wafer which is then in turnbonded to the wafer 30. If the light source is an edge emitting lightsource, a mirror 28 for directing the light is provided. An opticalelement 36 collimates the light. The coupler 12 then shapes the lightinto the desired profile. A further optical element 38 directs theshaped light onto a reflector 39, which in turn directs the shaped lightonto the optical fiber 14. Preferably, all of the optical elements areformed lithographically and all the elements are integrated on a waferlevel.

FIG. 9 illustrates the light source 10, the coupler 12 and the fiber 14mounted on an optical bench 40, preferably a silicon bench. The lightsource is preferably mounted on an electronics bench 42. The lightsource typically outputs a beam having a rapidly diverging fast axis anda more slowly diverging slow axis. A refractive element 44 having a highnumerical aperture, e.g., a gradient index lens, alters the divergenceangle along the fast axis such that, at the output of the high numericalaperture element, the fast axis has a divergence that is less than thedivergence angle of the slow axis. The refractive element is mounted ina v-groove 43.

A die 46 containing the coupler 12 is positioned a distance from therefractive element 44 such that a diameter of the beam incident thereonis substantially the same along both axes, i.e., the beam issubstantially circular. In a particular example, a collimating element48 is provided on the front surface of the die 46, with the coupler 12being on the back surface of the die 46. A lower plane is provided toaccommodate the diverging beam and the die 46 is mounted in a notch 47in the lower plane. The shaped beam output from the diffractive element12 is then delivered to the fiber 14 which is mounted in a v-groove 49in the bench 40, which may be a silicon wafer. Again, preferably all theoptical elements are formed lithographically and all the elements areintegrated on a wafer level.

FIG. 10 illustrates the light source 10, here a VCSEL, the coupler 12and the multi-mode fiber 14 integrated with a power monitor 64 and areflective surface 67 for directing the light into the fiber 14. Inparticular, the light source 10 and the power monitor 64 are provided ona substrate 60. Another substrate 66 has the coupler 12 thereon,preferably on the face furthest from the light source to allow the beamto expand, and a splitting diffractive element 61 which splits off aportion of the light from the light source 10 to be monitored. Thesubstrates 60, 66 are preferably mounted with spacer blocks 65, whichprovide the desired separation between the substrates 60, 66.

The light split off by the diffractive element 61 is directed to thepower monitor 64 to monitor the operation of the light source 10. Thedirected of the light to the power monitor 64 may be achieved byproviding appropriately positioned reflective portions. The number oftimes the light to be monitored traverses the substrate 66 is a designchoice, depending on the initial angle of diffraction and the desiredpositioning of the power monitor 64.

The light which is not split off by the diffractive element 61 proceedsto the coupler 12. A reflective surface 67, such as a polished angularface of another substrate, is provided to direct the light from thecoupler 12 into the multi-mode fiber. Again, preferably all the opticalelements are formed lithographically and all the elements are integratedon a wafer level.

While the above multi-mode coupler improves on previous couplers,additional performance may be realized by a third design, which uses adiffractive element to match the phase as well as the intensitydistribution of the beam. The matching of the phases generates spiralpropagation of the beam through the fiber. This spiral propagationmaintains the intensity profile input to the fiber along the fiber.Since the beam travels in a corkscrew, the amount of light crossing thecenter of the fiber is significantly reduced. For the purposes of thepresent invention, the center of the multi-mode fiber is considered tobe the region in which the refractive index is not sufficientlycontrolled such that differential mode delay arises.

Ideally, the intensity of light in the center will be zero, but inpractice, the intensity of light on axis is on the order of 10% of thepeak intensity. This reduced intensity is a substantial null. Incontrast, when only the intensity distribution is controlled, as in theother two designs, the input intensity profile may be the desiredprofile, but it well quickly degrade as the light traverses the fiber.In other words, while the other designs may provide an input profilewhich has a substantial null on axis, this profile is only maintainedfor the depth of focus of the coupler. When also matching the phase,this profile is maintained substantially beyond the depth of focus of alens having the same numerical aperture as the beam to be input to thefiber, e.g., at least an order of magnitude longer. Absent the fiber,the substantial null on axis of the beam is maintained through freespace, which significantly reduces the alignment requirement. Further,by matching the phase and amplitude of the beam to a certain mode of thefiber, theoretically the beam profile could be maintained over aninfinite length of fiber. However, imperfections in the real world,e.g., in the fiber, in the beam, in the matching, degrade from thistheoretical scenario. Nevertheless, with a matched phase and amplitudemode, the beam profile is maintained over a long distance in the fiber.

Thus, in order to avoid low order modes in a GRIN fiber launch, theamplitude and phase of the higher order modes need to be matched. Thefollowing equations are set forth in Fields and Waves in CommunicationElectronics, Simon Ramo et al. 1984, particularly pp. 765-768, which ishereby incorporated by reference in its entirety. For a GRIN fiber,these eigenmodes all have the form set forth in Equation (1):

E(r,φ,z)∝ƒ_(mp)(r)_(e) ^(±jmφ) _(e) ^(±jβ) ^(_(mp)) ^(z)  (1)

where ƒ(r) is a function that depends only on r for given modes within aspecific fiber, r is the radius from the axis, φ is the angle from theaxis, z is the distance along the axis, m is the azimuthal mode number,β is a propagation constant, p is the radial mode number. When m, p=0,the beam has a Gaussian profile.

While Equation (1) could be used to match a particular mode of the fiberby creating an input light beam having an amplitude and phase functionwhich exactly corresond to the particualr mode, such matching is notrequired. The azimuthal phase portion of Equation (1) is given byEquation (2). In order to supress the lowest order mode, i.e., m=0, aphase term needs to be added to the wave front. This is accomplishedthrough the use of the following diffractive phase function encoded ontothe wavefront set forth in Equation (2): $\begin{matrix}{{\varphi \left( {x,y} \right)} = {m\quad \arctan \quad \left( \frac{y}{x} \right)}} & (2)\end{matrix}$

where φ is the phase function, x and y are the coordinates in the plane.Any amplitude function may be used. As long as m>0 and the phasefunction is given by Equation (2), the phase function will providespiral propagation. Additionally, a substantial null at the center ofthe beam is created after having been phase matched after propagatingover a short distance, i.e., a few wavelengths. Unlike other types ofcoupling, this substantial null is maintained in the center in both freespace and the fiber, so such an optical element providing such a phasefunction does not have to be immediatley adjacent to the fiber.

In general, once the beam is in the fiber, there will be several modesexcited in the fiber. This multiple excitation will arise even if boththe phase and amplitude are matched, but will even more readily arisewhen only the phase function is provided by the optical element. Eachmode has a different intensity distribution, i.e., a differnt size ofthe substantial null on axis and a different width of the annulus. Thespiral mode may be realized, for example, by setting m=3 and providingthe phase function according to Equation (2).

This phase function can be added to a lens function and encoded as amod(2π) diffractive element as set forth in Equation (3):$\begin{matrix}{{\varphi \left( {x,y} \right)} = {\frac{\pi \left( {x^{2} + y^{2}} \right)}{\lambda \quad f} + {m\quad \arctan \quad \left( \frac{y}{x} \right)}}} & (3)\end{matrix}$

FIG. 11A illustrates the mod(2π) diffractive element and thecorresponding intensity in the focal plane of the lens function. FIG.11B illustrates an actual example of a spiral diffractive element orvortex lens 70 created in accordance with Equation (3). FIG. 11Cillustrates the simulated ring intensity 72 and the measured intensitypattern 74 of the vortex lens 70 in FIG. 11B. This vortex lens 70 may beused in place of the diffractive element 12 in any of the aboveconfigurations. A refractive equivalent in accordance with Equation (3)of the vortex lens 70 may be alternately employed.

This vortex lens 70 may be used in place of the optical element 12 inany of the previously discussed embodiments. This vortex lens 70 may beconsidered a beam shaper which directs light at a tangent azimuthally.The closer to the center of the vortex lens, the more light is directedaway from the center.

The vortex lens 70 allows the desired angular distribution to besusbtantially maintained along a portion of the fiber. This may bequantified by measuring the amount of power within a certain radius ofthe fiber at a certain distance along the fiber. The spiral phasefunction of the present invention allows more power to be containedwithin the desired radii for a longer distance than methods notemploying spiral propagation.

FIG. 12 is a plot of the the radius of the fiber and the encircledenergy encompassed thereby for a beam having a Gaussian profile and abeam having a spiral ring profile after propagating along a fiber for 6m. As can be seen therein, noting that the plot is cumulative, notincremental, more than half of the power of the light is within a 4micron radius and all of the power of the light is within 10 microns forthe Gaussian beam profile. Thus, for a Gaussian beam, no light isincident in a radius greater than 10 microns. In contrast, for thespiral ring profile, only a little over 60% of the light power isencircled in a 10 micron radius. Further, less than 10% of the lightpower is encircled by a radius of 4.5 microns. This compares favorablywith current requirements of 12.5% encircled power with a radius of 4.5microns, presently achieved using a VCSEL driven into a ring mode.

The ability of the beam having a spiral phase function imposed thereonto maintain a desired power distribution over a propogation distancerenders the vortex lens useful for other purposes as well as forcoupling to a mult-mode fiber. For example, any application which wouldbenefit from keeping power away from a center region of an object coulduse the vortex lens to achieve this goal over an extended propagationdistance.

The above different designs are useful in varying situations and thedifferent configurations shown in FIGS. 1A and 1B. For example, the useof a Fourier transform diffractive diffuser as the optical element 12 inFIG. 1B is not very practical, since there will not be enough room for aclean output shape, e.g., a ring to be formed from such a diffuser.However, the vortex lens 70 and the embodiments shown, for example, inFIGS. 4A, 4B and 5, including their diffractive diffusing patchequivalents, may be successfully employed with the configuration of FIG.1B as well as FIG. 1A. The configuration of FIG. 1B is advantageous,since it allows a conventional subassembly including a light source 10and a lens to be used with the optical element 12 placed close to thefiber 14.

Thus, it is evident that the coupler of the present invention is readilyincorporated into the light source/multi-mode fiber junction. Thecoupler of the present invention effectively and efficiently couples thelight from the light source to the fiber, allowing more light to becoupled to the fiber than would be absent the coupler.

By matching the phases, the light from the coupler is input to the fibertraveling in a spiral fashion, i.e., the path of the light down thefiber forms a corkscrew. Such traversal is opposed to the linear travelnormally occuring down the fiber. By traveling in a corkscrew or spiralmode, the input distribution, typically annular, of the input light ismaintained along the fiber. Without the spiral phase function, while theinitial input light has the desired shape, this shape is not retainedthroughout the traversal of the fiber. Therefore, more differntial modedelay will be present, with more light in the center of the fiber, ifspiral generation is not used.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

What is claimed is:
 1. An apparatus for coupling light from a lightsource into a optical fiber comprising an optical element for directinglight from the light source away from an undesired portion of theoptical fiber, wherein the optical element is an angular distributionaltering element and light output from the angular distribution alteringelement is distributed in a desired angular distribution.
 2. Theapparatus of claim 1, wherein the desired angular distribution is a ringpattern.
 3. The apparatus of claim 1, wherein the angular distributionaltering element Is different for a central portion of the light beamthan for a peripheral portion of the light beam.
 4. The apparatus ofclaim 1, wherein the angular distribution altering element is adiffractive element and the desired angular distribution is a multi-polepattern of series of N spots, where N is an integer and N≧1, distributedat locations r_(N) where spots in the output plane are located at r_(N)away from a center.
 5. The apparatus of claim 1, wherein the opticalelement is designed in accordance with a profile of a beam output by thelight source and desired modes of propagation for the optical fiber. 6.The apparatus of claim 1, wherein the optical element provides a firstangle of divergence for light in a central portion of a light beam fromthe light source and a second angle of divergence for light in aperipheral portion of the light beam, the first angle being greater thanthe second angle.
 7. The apparatus of claim 1, wherein the opticalelement is provided in a housing which receives the optical fiber. 8.The apparatus of claim 1, wherein the optical element islithographically created on a wafer level.
 9. The apparatus of claim 1,further comprising an optical bench including mounts for the lightsource, the fiber and the optical element.
 10. The apparatus of claim 1,wherein the optical element incorporates further optical functioningtherein.
 11. The apparatus of claim 1, wherein the optical element is adiffractive diffusing element.
 12. The apparatus of claim 11, whereinlight output from the diffractive diffusing element is distributed in aring pattern.
 13. The apparatus of claim 1, wherein the optical fiber isa multi-mode GRIN fiber and the optical element couples light intohigher order modes of the multi-mode GRIN fiber to reduce differentialmode delay.
 14. The apparatus of claim 1, further comprising a beamshaping element in front of the optical element.
 15. The apparatus ofclaim 14, wherein the beam shaping element outputs a beam having asubstantially uniform peak.
 16. The apparatus of claim 14, wherein thebeam shaping element outputs a beam having a substantially uniform peak.17. An apparatus for coupling light from a light source into a opticalfiber comprising an optical element for directing light from the lightsource away from an undesired portion of the optical fiber, whereinlight output from the optical element is distributed in a desiredangular distribution which is substantially maintained along an axis ofpropogation for more than a depth of focus of the optical element. 18.The apparatus of claim 17, wherein light the desired angulardistribution includes a null on the axis of propagation.
 19. Theapparatus of claim 17, wherein the optical element is a diffractivevortex lens.
 20. The apparatus of claim 17, wherein the optical elementis a refractive vortex lens.
 21. The apparatus of claim 17, wherein theoptical element is created in accordance with the following equation:${\varphi \left( {x,y} \right)} = {m\quad \arctan \quad \left( \frac{y}{x} \right)}$

where φ is a phase function, x,y are positions in the plane, and m is anazimuthal mode number.
 22. The apparatus of claim 17, wherein theoptical element includes a lens function and is created in accordancewith the following equation:${\varphi \left( {x,y} \right)} = {\frac{\pi \left( {x^{2} + y^{2}} \right)}{\lambda \quad f} + {m\quad \arctan \quad \left( \frac{y}{x} \right)}}$

where φ is a phase function, x,y are positions in the plane, m is anazimuthal mode number and λ is a wavelength of the light source.
 23. Asystem for coupling light to an optical fiber comprising: a lightsource; and an optical element for directing light from the light sourceaway from a center of the optical fiber, wherein the optical element isan angular distribution altering element.
 24. The apparatus of claim 23,wherein the system is provided in a housing with the optical fiber. 25.The system of claim 23, wherein the system is integrated on a waferlevel.
 26. The system of claim 23, wherein the light output from theangular distribution altering element is distributed in a ring pattern.27. The system of claim 23, wherein the angular distribution alteringelement is provided only for a central portion of the light beam. 28.The system of claim 23, wherein the angular distribution alteringelement is a diffractive diffusing element.
 29. The system of claim 23,wherein the optical element is designed in accordance with a profile ofa beam output by the light source and desired modes of propagation forthe optical fiber.
 30. The system of claim 23, wherein the opticalelement provides a first angle of divergence for light in a centralportion of a light beam from the light source and a second angle ofdivergence for light in a peripheral portion of the light beam, thefirst angle being greater than the second angle.
 31. The system of claim23, wherein the optical element is a vortex lens, wherein light outputfrom the optical element is distributed in a desired angulardistribution which is substantially maintained along the fiber for morethan a depth of focus of the optical element.
 32. A method for couplinglight into an optical fiber comprising: generating a light beam; andusing an optical element for directing light from the light source awayfrom a center of the optical fiber, wherein said directing furthercomprises adding a phase term to a wavefront of the light beam to matchhigher order modes of the fiber.
 33. The method of claim 32, whereinsaid directing comprises altering an angular distribution of the beam tobe distributed in a ring pattern.
 34. A method for coupling light intoan optical fiber comprising: generating a light beam; and using anoptical element for directing light from the light source away from acenter of the optical fiber, wherein said directing comprises shapingthe beam and matching modes to provide light in a desired pattern andsubstantially maintain the desired pattern along an axis of propagationfor more than a depth of focus of the optical element.
 35. An apparatusfor coupling light from a light source into a optical fiber comprisingan optical element for directing light from the light source away froman undesired portion of the optical fiber, wherein the optical elementis designed in accordance with a profile of a beam output by the lightsource and desired modes of propagation for the optical fiber.
 36. Anapparatus for coupling light from a light source into a optical fibercomprising an optical element for directing light from the light sourceaway from an undesired portion of the optical fiber, wherein the opticalelement is provided in a housing which receives the optical fiber. 37.An apparatus for coupling light from a light source into a optical fibercomprising an optical element for directing light from the light sourceaway from an undesired portion of the optical fiber, wherein the opticalelement is lithographically created on a wafer level.
 38. An apparatusfor coupling light from a light source into a optical fiber comprisingan optical element for directing light from the light source away froman undesired portion of the optical fiber, further comprising an opticalbench including mounts for the light source, the fiber and the opticalelement.
 39. An apparatus for coupling light from a light source into aoptical fiber comprising an optical element for directing light from thelight source away from an undesired portion of the optical fiber,wherein the optical element incorporates further optical functioningtherein.
 40. An apparatus for coupling light from a light source into aoptical fiber comprising an optical element for directing light from thelight source away from an undesired portion of the optical fiber,wherein the optical fiber is a multi-mode GRIN fiber and the opticalelement couples light into higher order modes of the multi-mode GRINfiber to reduce differential mode delay.
 41. An apparatus for couplinglight from a light source into a optical fiber comprising: an opticalelement for directing light from the light source away from an undesiredportion of the optical fiber; and a beam shaping element in front of theoptical element.